1. Field of the Invention
The invention relates to an apparatus for infusion of liquids and somewhat more particularly to a fail-safe apparatus for infusion of liquid medications into human or animal bodies.
2. Prior Art
In the treatment of patients, whether human or animal, with liquid medications, the medications must be delivered from a reservoir through an outflow catheter in such a manner that the amount of liquid infused per unit time can be accurately dosed and controlled. This is particularly acute in instances where, for example, in the treatment of diabetes, insulin must be continuously infused at different controllable rates because the insulin demand of a diabetic may be subject to considerable fluctuations during the course of any one day. Such fluctuations may be caused, for example, because of a variable frequency of meal times. For an insulin concentration of, for example, 500 IE/ml in the infusion liquid, catheter flow rates in the order of microliters per hour (for example, 10.sup.-6 1/hr), are necessary. A daily dose for a given patient may correspond to about 1 to 2 drops of infusion liquid.
Presently known devices for infusion of liquids into human or animal bodies generally comprise a reservoir for the infusion liquid and a controllable delivery-dosing means coupled thereto for delivering the liquid from the reservoir to the outflow opening of an outflow catheter. Such devices may be implanted in the body of a patient or may be worn outside the body. Typically, the prior art devices utilize a combination of an excess pressure and a flow resistance as the delivery-dosing means. In such devices, a constant flow, controllable by valves, is produced by the action of the excess pressure on the volume of liquid through-flow in the resistance, for example, in the form of a capillary. By connecting two flow resistances in parallel, some of these devices attain a constant basal rate with a periodically activatable peak rate. However, the use of valves in association with an excess pressure producing means, particularly when applied to human bodies, is dangerous or at least disadvantageous because if there is a failure of the valves, the flow of medication into the body is not stopped. For example, in the treatment of diabetes, an overdose of insulin can cause serious damage to a patient. Accordingly, in order to insure optimum patient safety, there is a need for an infusion device capable of delivering only a particular volume of liquid from a reservoir to the outflow catheter despite any mechanical failure or malfunction.